The present invention pertains to signalling or communication sub-systems and particularly to such systems such as track circuits, speed governors and vital communication links which function to control a vital relay whose contacts are utilized in so-called vital circuits.
As background for the present invention reference may be made to sub-systems of the type described in U.S. Pat. No. 3,955,125, assigned to the assignee of the present invention, involving signalling or communication circuits adapted to be used with railroad tracks or the like for insuring fail-safe performance in the operation of certain critical control devices. Such sub-systems include the operation of a vital or fail-safe relay driver functioning to control a vital relay, which relay is so designed that when there is failure of current, the relay acts to open its contacts so as to insure a safe action. In other words, in the given sub-systems the vital relay is normally energized so long as a safe situation obtains, but this relay will be released without fail when the situation becomes unsafe. These sub-systems generally operate in a cyclic manner so that a finite time is required to determine if the situation is indeed safe. The vital relay when energized must remain energized during the time that the system is making a new determination of safety. As long as the safe situation continues, the relay remains energized with no interruptions. However, as soon as an unsafe situation is discovered, the relay must be released quickly.
A fundamental problem exists in the situation present, for example, in the case of vehicle headways where, if the release time of the relay is not held to close tolerance, its worst case maximum time must be considered in safety design, thereby imposing a penalty on the involved vehicle headway. Moreover, in certain kinds of communication systems, it is desirable to be able to select the vital relay release time so as to be appropriate for the given bit rate, channel noise and response time requirements of the system. For example, in a speed governor, it is very desirable to be able to select a short release time when the vehicle speed is high, but to be able to lengthen that release time when the speed measurement requires a greater time interval for measurement and a fast response time is not required.
Vital relay drivers currently employed, one of which may be appreciated by reference to the aforesaid patent, operate to rectify a dynamic signal, often of a very low frequency, and to filter the resultant direct current with large capacitors. Such capacitors are almost invariably electrolytic in nature, they have large tolerances, and they become less effective at very low temperatures. Also, the relay drivers are generally required to operate from an unregulated battery power supply. Consequently, the capacitor values are chosen to insure that the relay does not release falsely when the temperature is at its lowest extreme and the battery voltage has dropped to its minimum. In actual use, the temperature is generally moderate and the batteries are fully charged producing the longest possible response time.
Accordingly, as a result of the aforenoted drawbacks and disadvantages, it turns out that it is not easy to change the response or release time of these vital drivers as conditions warrant. In addition there is the expense of vitally switching components in and out of the circuit.
It is therefore a primary object of the present invention to provide a vital relay driver which has flexibility with respect to the release times that can be selected to suit varying conditions, such release or sustain times being held to close tolerances by means of digital techniques.
The above objects are distinguished from those fulfilled, for example, by the features specifically detailed in U.S. Pat. No. 3,794,834 in which an automatic vehicle control system is described for controlling the operation of a vehicle from a set of parallel operating computers in a central control station, and in which a data communication link transmits vehicle performance reports from the vehicle to the central control station and transmits vehicle command messages from the central control station to the vehicle.
The important distinguishing point to be emphasized with relation to U.S. Pat. No. 3,794,834 is that, contrasted with that system or arrangement, the present invention involves a vital relay driver which does not determine whether or not the relay should be driven, such determination being made elsewhere, in the form of vital message decoders or speed governors, or the like. The vital relay driver's sole function is to enforce the decision that has been made on the relay. Also in the aforenoted U.S. Pat. No. 3,794,834, release times of any relays are determined by hardware parameters and could not be changed by a computer program.